Assumptions are of a standard new clutch disc being .280" thick, which would very quickly bed in to .270".
I also assume a minimum of .170" thick, which would be just down to the head of the rivet, and will be starting to wear the peak off the splayed ends of the rivet. You'd likely have changed the clutch by this point hopefully!

Starting with the Grey. This clutch is obviously the easiest to evaluate because there is no lever ratio - it is a simple belville spring after all. In new condition, when set up properly with a new plate, can have a peak clamp force of 920lb, and a release force of 960lb.
When worn, this is crazy different, as the force has actually dropped off way too much with .10" of wear, which is why the pre-verto's seem to only clamp well for a short period of time. Basically, you need to be setting it up perfectly, and then changing the disc out probably after just .040" (1mm) of wear.
The release force when worn by any margin, seems to be always just a little more than the clamp force at that time (which is obvious if you think about it).

Verto Turbo (2.42mm thick spring)
This clutch seems to pretty much be a 2:1 ratio, but the actual numbers dimensionally would not suggest this... Sadly, I didn't measure with a worn, as I took the data a long time ago, then cut it up to make an RTS for evaluation. Anyway, it had a peak clamp force of 820lb, but is able to transmit over 500lb clamp force over 0.10" - which means although it is less peak than the grey verto, it doesn't "fall off" as quickly as the verto. Release force is up around 320lb, but I didn't measure how much it increases with a worn plate.

RTS (2.42mm and 2.20mm thick springs)
I spent a long time tweaking, and adjusting this, doing a lot of fine-tuning to get it dialled in to the optimum. One conclusion I have come to, is that it's interesting the difference in how the springs are stacked; one takes more travel to release than the other, because you've pushed down on the "top" spring, but the bottom one is the one acting against the clutch disc, so you need to over-travel the top spring as they start to "seperate" at the outer edge, leaving the inner one still holding the clutch (with reduced force) until you've added more travel. I'd reccomend trying your RTS build up with the springs stacked both ways, and look for the least seperation of the outer edges of springs, and use that setup. Remember to test wioth both a .280 and a .180 (or maybe just .200") thick spacers simulating a new and worn clutch to evaluate both stack options.

Anyways, I was able to get a peak of 1450lb clamp force, and most importantly, it could hold 1100lb clamp force over 0.10" and 1400lb clamp force over .040" (1mm).
Obviously, you need to take the data and decide if you want 1400lb with a new plate, and accept that if it wears .065" that you'll be down to less than 900lb clamp force, or (what I optimised) that you start at 1100lb with a new plate, which rises to a peak of 1450lb with .060" wear, then drops to 1100lb after .010" wear.

To get that wear pattern, I found that the post heights need to be 21.7mm, instead of the 23.5mm of the standard setup incidentally...
The thing I really am not keen on, is the release force with the RTS - it's 435lb with a brand-new clutch, and rises to a whopping 670lb with a worn clutch. That's almost orange release force...

How do you guys even manage to push the pedal with a RTS; are you lengthening the arm to play with the lever ratios?

Finally, the 3S clutch - or the Super Single Spring as I've christened it LOL
This is basically using the original clutch components (with the exception of the spring) but uses a different spring of a larger diameter, that sits on a new cover assy, bolted to the outside of the clutch.
Peak clamp force is basically the same as the killer RTS setup, 1100lb over 0.10", but a slightly higher peak of 1500lb. The part I do like is the release force of 250lb new, or 400lb fully worn...

I'm making a new cover for the 3S clutch, as the one I've got has been butchered and shimmed, and I'm not happy putting it in a vehicle for testing... Hopefully, later this summer! In the meantime, I'll be testing the RTS, but I'm already thinking I need to adjust the arm lever ratio...

I thought the greys were supposed to be more clamping force than that, like 1100lb I know its quoted in hamils book, maybe im thinking of the double grey.

1500lb is around the amount i guestimated for an RTS with an iffy spread sheed from roberts old numbers, and i do seem to remember estimating the relase as about the same as an orange plate.
in terms of pedal pressure, they can be hard going, although they do seem to bed in a fair bit, I presume this is due to stiction between the springs, no idea how many times your tests were repeated, maybe not enough to "bed in". also on the ones I have done I have lubed between them with graphite (yeah i couloured it in with a 2B pencil!) which seemed to help, and also i did move to the longer metro cable arm.

The new clutch spring sounds damn interesting, I did read somthing a while back about moving the pivit points on the std clutch to make averything work better, but there were no details, I think it was Matt Read that was doing it.

The fact that you had to extend the arm, makes it clear that the release load is "up there"...

I'm assuming you also need the full travel of your pedal as the clutch wears (a LOT more travel needed at release bearing as it wears). I know I found many years ago there was a benefit to lengthening the clutch master pushrod to raise the pedal, so more travel was available at the master cylinder. I feel like the RTS definitely needs all the travel that can be mustered when worn; especially if you're altering the clutch arm lever ratio - which reduces the travel at the release bearing. Lengthening the master pushrod will enable more travel - up to the limitations of the master / slave cylinders of course!

I have measured a double grey before, and have the data. 1100lb does sound familiar. I'll update the post above later.

EDIT I did try the RTS with a little grease, and also totally dry. I saw no difference in dynamic force or release force, but did see a smoother, more linear operation (dry it would change linearly in distance on the DTI, and "jump" in 10-30lb increments on force). Bear in mind though, I'm moving VERY slowly, in 100lb steps, then taking three readings of distance (averaged out each time). Perhaps your feeling of it bedding in, reducing the pedal force, is a placebo based on your left leg getting stronger?

Yes, extending the arm for the RTS is the way to go for a reasonable pedal. The trouble with the actual Metro one is the angle of the ends is all wrong so the slave has to be re-located but, having said that, even if you just extend a Mini verto arm the slave has to be raised to keep the pushrod parallel to the cylinder.

And to counteract the loss of motion at the slave cylinder end, extend the master cylinder pushrod so the pedal starts from further up - not that strange, lots of modern cars have different height clutch and brake pedals (apart from the USA where one is missing completely.....).
I did all the detailed measurements on a spare pedal box and a dis-assembled master cylinder and there is a fair bit of extra travel available in the master cylinder without any issues with the porting.

My RTS isn't in yet (waiting for next time the engine is out) but Graham T and I bought the same extended master cylinder pushrod at the same time and I've tried his pedal and it's lighter than my current single grey.

One other trick I've added - again, yet to be proven - is a 0.5mm sheet of PTFE cut into a circle with "fingers just like the springs, between the springs.
Same principle as the lead pencil trick but hopefully longer lasting.

PTFE will withstand very high loads and quite high temperatures, in my industry we use it for the keyway slides and expansion guides on pretty large steam turbines although the stuff I bought on eBay may not quite match the power generation industry spec. Time will tell, I may just end up with a gooey sticky mess...

well my RTS was never over the top weight wise, IIRC its 2x 1275 springs, but theres people out there running 2x turbo springs and that would be a bit hard going!

i was using mine on the road, and going tot he longer arm was mainly because i have to fuck with stuff even if it dosent need it, but it did giove a nice decrease in effort and put the bite point down the travel a bit, and jsut gererally felt better, more by luck than judgement!

was there a peak release force for the grey? or was the 960lb it, even worst case with the RTS, its still a lot better on the thrusts, even if not on the knees

This "3S" clutch, is that Verto or Preverto based??

also do we know what the MED and Mispares lightweight verto post heights are? I think its mentioned somewhere, i'll have a dig later if no one knwos off hand.

also do we know what the MED and Mispares lightweight verto post heights are? I think its mentioned somewhere, i'll have a dig later if no one knwos off hand.

The 3S is verto based. My old flywheel design I was working on back in 2006 (using the same 3S spring) basically came full circle, and ended up looking like a verto, so I re-designed it around the verto stuff.

The MS Verto lightweight post heights are same as standard - 23.5mm

I found they needed to be reduced to 21.7mm for "my" optimal clamping preference. The dowels need to be removed to machine the posts, and (on my MS plate) they are in there TIGHT. I had to put a 3/8 nut over them, plug weld, and twist and pry to get them out. They are simple 8mm dowels however.

As i mentioned, I had 2.4 and 2.2mm springs - I believe that is a turbo and a 1275.

One other trick I've added - again, yet to be proven - is a 0.5mm sheet of PTFE cut into a circle with "fingers just like the springs, between the springs.
Same principle as the lead pencil trick but hopefully longer lasting.

That there is a good idea.
Yes, PTFE takes high temps for sure, we use UHMW on all conveyors, but switch to PTFE in the oven conveyors. I imagine that polishing up the faces will also be of benefit. I feel like the lubricant alone will not make a huge difference, as it's simply going to contact on the highest (rough) points of the rough metal springs, and that pressure (and surface rougnmess will negate the dry (graphite) or wet (grease) lubricant.

The PTFE will definitely aid compliance. I'm also wondering about cutting pockets into the area between the fulcrum pivots (the wire rings) leaving an assured full contact at the wire and outer perimeter, but allow the other areas to move closer (if they need).

As much as you're measuring at that clutch thickness ( I actually thought a new disc was thicker ? but it's been near 20 years since I measured one )...

Have you tried to see at what cover position offers the most clamp load ? ie would a thicker or thinner starting disc be better, or a height adjustment needed on the flywheel posts ?

Also...and I've said it before, quality between verto's just seems to be a stab in the dark.
I went through about 7-8 clutches before I found one that would hold on my car back then.
My simple test was picking a cover off the shelf, sticking the cup over the fingers and standing on it ! lol. When I found the stiffest one I used it. And it worked.

I'd went through about 3-4 turbo clutches including factory items and a few aftermarket vertos to no avail. So went to a factors that had a few on the shelf and tried them all.
And the strongest one I found was just a 1000cc Mini/Metro clutch by Balco.
The same cover is still in the car today and I used it from 94-98 when the car was driven and it did see some 70-80k.
The same cover....with several clutch plates and a few new friction surfaces riveted to that same cover due to wear.

All the other clutches would slip almost immediately I gave it any boost. This one held with an AP paddle disc to over 20psi without issue ( "big" T3 )

On 30th Mar, 2018 stevieturbo said:As much as you're measuring at that clutch thickness ( I actually thought a new disc was thicker ? but it's been near 20 years since I measured one )...

Have you tried to see at what cover position offers the most clamp load ? ie would a thicker or thinner starting disc be better, or a height adjustment needed on the flywheel posts ?

I measured the AP race/rally plate. When squeezed together, it is indeed .280" (7.1mm).It is thicker when not squeezed, of course, but that is not representative.

I have indeed played with the post height. This pic shows the clamp force. The curve goes from the left to the right for the spring relaxing - that is to say a worn clutch would be on the right, a new clutch on the left. I adjusted my posts so at .280" thick, it'll be at the left green marker, and as it wears, clamp force increases, then decreases. I could instead have set it up to be at the peak at .280", but as you can see, it'd fall off the peak clamp quickly...

The MED flywheel I had, needed the posts machining down to get the springs flat, but the standard MPi flywheel was spot on. The after market flywheels boast “greater clamping” but couldn’t get my head round that. It’s basically bullshit. What they have done is put the spring into its peak clamping force region.

On 26th Oct, 2004 TurboDave16v said: Is it A-Series only? I think it should be...
So when some joey comes on here about how his 16v turbo vauxhall is great compared to ours, he can be given the 'bird'...

Well they probably are claiming that because they realised (as I did) that the springs flat point, is not the greatest clamping force.

In my analysis, I didn’t assume spring was offering highest force when spring was flat; in fact I didn’t look at the spring. I supported the verto drive plate on my press, upside down, at the six locations of the bolt heads, representative of real-life.
I then pressed down on the back plate (simulating the clutch plate), measuring the force, and the distance (over the average of three points) to get a force/displacement reading between the face at the six mounting points where the main drive plate contacts the flywheel. That then allows me to machine the flywheel post heights to the exact height that I want (knowing the compressed clutch plate thickness).

Actually, I should add that I started by fully compressing, past the peak (spring past flat) and then retracting the ram, so the force is measured that way. The results you get doing it this way are lower, but fully representative. That’s why the data above starts at a set point (spring over centre) and comes back to zero (although I stopped at 900lb).

I did not standardize around the spring flatness, I didn’t even think to look at it. But after it was finally built; I realised that the spring(s) was actually perfectly flat, meaning peak force is when the spring is about .060” below flat.

I think plate thickness ( as new ) is an issue that gets overlooked, Ive always used the 190mm rally plate, and IIRC its a fair bit thicker, more like 7.8-8 mm from memory. whioch i think puts it in a more favorable position?

On 30th Mar, 2018 TurboDave16V said:Well they probably are claiming that because they realised (as I did) that the springs flat point, is not the greatest clamping force.

I think there is some misunderstanding about the diaphragm, and probably only recently learned it myself.

I had previously thought a clutch would install pretty close to flat...and that the release mech would push to flat and beyond to disengage drive.

Seems that is nowhere near correct.

Clutch fingers installed should not be flat...and indeed nor should they ever go flat or beyond even to disengage. The spring should always remain less than flat, going beyond will stress and wear the spring more quickly. The diaphragm should never go over centre apparently.

So as I'm sure Dave noted....max clamp will actually be some distance prior to flat fingers

I tested my ACT cover a few months back and was surprised at where max clamp actually was, as I thought because my disc pack was worn to the tune of around 1mm total ( twin plate )....this was the reason it started slipping on me and removing shims from the posts might restore clamp.
But if anything at that "worn" installed height, clamping force on the discs would be higher than anywhere else and the clutch fingers were at a fair angle. It just wasnt strong enough in my case.